The objective of this grant proposal is to identify the plasma lipoprotein source of cholesterol utilized by the liver for bile acid synthesis and biliary cholesterol secretion in man. This is a natural extension of our previous finding that the total plasma free compartment is the major source of the cholesterol present in the bile acid and biliary cholesterol precursor sites. Experiments will be carried out in subjects administered pairs of autologous lipoproteins labeled with free cholesterol (for example, 3H-HDL/14C-LDL) by a new method developed in this laboratory. In other experiments 3H labeled cholesterol esters in a specific lipoprotein class will be administered. Using this experimental approach, the free and esterified cholesterol from each lipoprotein class will be systematically tested as potential precursors of the biliary sterols. Multicompartmental analysis of the radioactivity data using the SAAM-27 computer program will give quantitative values (fluxes) for the pathways between potential lipoprotein cholesterol sources and hepatic compartments for bile acids and biliary cholesterol. Critical information will be obtained for the first time in man on the role of the liver in lipoprotein degradation and on lipoprotein free cholesterol metabolism. A preliminary study suggested that HDL free cholesterol and not the HDL esterified cholesterol or LDL free cholesterol, is preferred by the liver for bile acid synthesis and biliary cholesterol secretion. By promoting cholesterol excretion, high HDL levels would tend to keep tissue cholesterol content low and may explain the inverse relationships between plasma HDL concentration and the incidence of coronary artery disease. It is also planned to investigate the mechanism of uptake of lipoprotein free cholesterol by the hepatocyte (receptor mediated?); the perfused swine liver will be used to investigate aspects of hepatic-lipoprotein cholesterol metabolism which cannot be studied in vivo in man. The studies outlined in this grant proposal will provide basic information on cholesterol metabolism in man, and should give insight to the disorders of atherosclerosis and cholesterol gallstone formation.